The present invention relates to radio wave antennas, and, more particularly, to deployable antennas including tensegrity support architecture.
The field of deployable structures, such as space-deployed platforms, has matured significantly in the past decade. What once was a difficult art to master has been developed into a number of practical applications by commercial enterprises. The significance of this maturity has been the reliable deployment of various spacecraft-supported antenna systems, similar to that employed by the NASA tracking data and relay satellite (TDRS). In recent years, the development of parabolic, mesh-surface, reflector geometries has been accompanied by improvements in phased arrays (flat panel structures with electronically steered beams), both of which are critical to commercial and defense space programs. As commercial spacecraft production has exceeded military/civil applications, there is currently a demand for structural systems with proven reliability and performance, and the ever present reduced cost.
The mission objective for a large, deployable space antenna is to provide reliable radio frequency (RF) energy reflection to an electronic collector (feed) located at the focus of the parabolic surface. The current state of deployable parabolic space antenna design is principally based on what may be termed a segmented construction approach which is configured much like an umbrella. In this type of design, a plurality of radial ribs or segments are connected to a central hub, that supports an antenna feed. A mechanical advantaged linear actuator is used to drive the segments from their stowed or unfurled condition into a locked, over-driven, position, so as to deploy a surface. A shortcoming of a single fold design of this type of antenna is the fact that the height of the stowed package is over one half of the deployed diameter. Other proposals include the use of hoop tensioners and mechanical memory surface materials.
In recent years, numerous Defense Department organizations have solicited for new approaches to deployable antenna structures. The Air Force Research Laboratories (AFRL) are interested in solutions to aid with their Space Based Laser and Radar programs, and have requested new solutions to building precision deployable structures to support the optical and radar payloads. These requests are based upon the premise that the stowed density for deployable antennas can be significantly increased, while maintaining the reliability that the space community has enjoyed in the past. A failure of these structures is unacceptable. If the stowed volume can be reduced (therefore an increase in density for a given weight), launch services can be applied more efficiently.
The implementation of multiple vehicle launch platforms (e.g., the Iridium satellite built by Motorola) has presented a new case where the launch efficiency is a function of the stowed spacecraft package, and not the weight of the electronic bus. For extremely high frequency (EHF) systems (greater than 20 GHz) in low earth orbit (LEO), the antenna aperture needs to be only a few meters in diameter. However, for an L-band, geosynchronous orbit satellite (such AceS built by Lockheed Martin), the antenna aperture diameter is fifty feet. Less weight and payload drag must be achieved to ensure a more efficient assent into a geosynchronous orbit.
Tetrobots have been developed in the last few years as a new approach to modular design. The tetrobot approach, which is described in the text by G. Hamlin et al, entitled: xe2x80x9cTETROBOT, A Modular Approach to Reconfigurable Parallel Robotics,xe2x80x9d Kluwer Academic Publishers, 1998 (ISBN: 0-7923-8025-8) utilizes a system of hardware components, algorithms, and software to build various robotic structures to meet multiple design needs. These structures are Platonic Solids (tetrahedral and octahedral modules), with all the connections made with truss members. Adaptive trusses have been applied to the field of deployable structures, providing the greatest stiffness and strength for a given weight of any articulated structure or mechanism.
The most complex issue in developing a reliable deployable structure design is the packaging of a light weight subsystem in as small a volume as possible, while ensuring that the deployed structure meets system requirements and mission performance. An article by D. Warnaar, entitled: Evaluation Criteria for Conceptual of Deployable-Foldable Truss Structures,xe2x80x9d ASME Design Engineering: Mechanical Design and Synthesis, Vol. 46, pp. 167-173, 1992, in describing criteria developed for deployable-foldable truss structures, addresses the issues of conceptual design, storage space, structural mass, structural integrity, and deployment. This article simplifies the concepts related to a stowed two-dimensional area deploying to a three-dimensional volume. A tutorial on deployable-foldable truss structures is presented in: xe2x80x9cConceptual Design of Deployable-Foldable Truss Structures Using Graph Theory-Part 1: Graph Generation,xe2x80x9d by D. Warnaar et al, ASME 1990 Mechanisms Conference, pp. 107-113, September 1990, and xe2x80x9cConceptual Design of Deployable-Foldable Truss Structures Using Graph Theory-Part 2: Generation of Deployable Truss Module Design Concepts, by D. Warnaar et al, ASME, 1990 Mechanisms Conference, pp. 115-125, September 1990. This series of algorithms presents a mathematical representation for the folded (three-dimensional volume in a two-dimensional area) truss, and aids in determining the various combinations for a folded truss design.
NASA""s Langley Research Center has extensive experience in developing truss structures for space. One application, a 14-meter diameter, three-ring optical truss, was designed for space observation missions. An article by K. Wu et al, entitled: xe2x80x9cMulticriterion Preliminary Design of a Tetrahedral Truss Platform,xe2x80x9d Journal of Spacecraft and Rockets, Vol. 33, No. 3, May-June 1996, pp. 410-415, details a design study that was performed using the Taguchi methods to define key parameters for a Pareto-optimal design: maximum structural frequency, minimum mass, and the maximum frequency to mass ratio. In the study, tetrahedral cells were used for the structure between two precision surfaces. 31 analyses were performed on 19,683 possible designs with an average frequency-to-mass ratio between 0.11 and 0.13 Hz/kg. This results in an impressive 22 to 26 Hz for a 200-kg structure.
The field of deployable space structures has proven to be both technically challenging and financially lucrative during the last few decades. Such applications as large parabolic antennas require extensive experience and tooling to develop, but is a key component to the growing personal communications market. Patents relating to deployable space structures have typically focused on the deployment of general truss network designs, rather than specific antenna designs. Some of these patents address new approaches that have not been seen in publication.
For example, the U.S. patents to Kaplan et al, U.S. Pat. No. 4,030,102, and Waters et al, U.S. Pat. No. 4,825,225 describe the application of strut and tie construction to deployable antennas. However, the majority of patents address trusses and the issues associated with their deployment and minimal stowage volume. For example, the U.S. patent to Nelson U.S. Pat. No. 4,539,786 describes a design for a three-dimensional rectangular volume based on an octahedron. Deployment uses a series of ties within the truss network, and details of the joints and hinges are described. When networked with other octahedral subsets, a compact stow package could be expanded into a rigid three-dimensional framework.
Other patents describe continued work in expandable networks to meet the needs of International Space Station. For example, the U.S. patent to Natori U.S. Pat. No. 4,655,022, employs beams and triangular plates to form tetrahedral units that provide a linear truss. The patent details both joint and hinge details and the stowage and deployment kinematics. Similarly, the U.S. patent to Kitamura et al, U.S. Pat. No. 5,085,018, describes a design based on triangular plates, hinged cross members, and ties to build expanding masts from very small packages.
A series of U.S. patents to Onoda, U.S. Pat. Nos. 4,667,451, 4,745,725, 4,771,585, 4,819,399 and 5,040,349 and an article by Onoda et al, entitled: xe2x80x9cTwo-Dimensional Deployable Hexapod Truss,xe2x80x9d Journal of Spacecraft and Rockets, Vol. 33, No. 3, May-June 1996, pp. 416-421, detail a number of examples of collapsible/deployable square truss units using struts and ties. Some suggested applications included box section, curved frames for building solar reflectors or antennas. The U.S. patent to M. Rhodes et al, U.S. Pat. No. 5,016,418, describes a more practical design that uses no ties, but employs hinges to build a rectangular box from a tube stowage volume. In addition, the U.S. patents to Krishnapillai, U.S. Pat. No. 5,167,100 and Skelton, U.S. Pat. No. 5,642,590, describe the use of radial struts and strut/tie combinations, respectively.
An article by B. F. Knight et al. entitled: xe2x80x9cInnovative Deployable Antenna Developments Using Tensegrity Design,xe2x80x9d Abstract for the 41st AIAA Structures Conference, April 2000, and an article by A. Hoover entitled: xe2x80x9cFor 21st-Century Campers and Soldiers, A Tent That Sets Itself Up,xe2x80x9d UF News, December 1999, page 30, describe the potential use of tensegrity support structures for antennas, but offer no teaching of how to integrate or mount a reflector membrane, e.g. a reflector mesh, to the structure to ensure proper deployment in the desired antenna operating shape.
Thus, there is a desire to provide a mounting device and method for mounting a reflector to a tensegrity support structure to ensure proper deployment of the reflector in the desired antenna operating shape.
In view of the foregoing background, it is therefore an object of the invention to provide a reflector assembly or antenna using tensegrity support architecture with a mounting frame for connection to and reliable deployment of the reflector.
This and other objects, features and advantages in accordance with the present invention are provided by a reflector assembly or antenna including a tensegrity structure having a plurality of compression members and a plurality of tension members connected thereto. The tensegrity structure is movable between stored and deployed positions. The assembly includes an actuator for selectively moving the tensegrity structure to the deployed position, a reflective member, such as an electrically conductive mesh, movable to an operating shape, and a mounting frame for connecting the reflective member to the tensegrity structure so that the reflective member is in the operating shape when the tensegrity structure is in the deployed position. The operating shape is preferably a parabolic dish.
The mounting frame may include a plurality of base members carried by the tensegrity structure, and a plurality of hanger members connected between the base members and the relective member. Furthermore, each of the base members and each of the hanger members preferably comprise a flexible elongate member.
In one embodiment, the plurality of base members may comprise a plurality of primary base members connected to the tensegrity structure, and a plurality of secondary base members connected between primary base members. The hanger members are connected between the secondary base members and the relective member. The secondary base members are arranged in a plurality of spaced apart sets, each set defining a polygonal shape, and each successive set defining a reduced area polygonal shape. Also, each compression and tension member has an elongate shape with opposing ends with respective adjacent ends of the compression and tension members defining a node of the tensegrity structure therebetween. Each primary base member has an elongate shape and opposing ends connected to respective nodes of the tensegrity structure.
Each base member has an elongate shape and opposing ends connected to respective compression members or tension members along medial portions thereof. Each of the compression members is preferably rigid, and each of the tension members is preferably flexible. Also, the actuator preferably selectively moves the tensegrity structure to the deployed position via a screw motion. Of course an antenna feed may be provided adjacent the tensegrity structure for receiving radio waves reflected from the reflective member and transmitting radio waves to the reflective member.
Objects, features and advantages in accordance with the present invention are also provided by a method of deploying a mesh reflector antenna including providing a reflective member, such as an electrically conductive mesh, movable to an operating shape, such as a parabolic dish, and connecting the electrically conductive reflector to a tensegrity structure via a mounting frame. Again, the tensegrity structure is deployable from a stored position to a deployed position and comprises a plurality of compression members and a plurality of tension members connected thereto. The mounting frame connects the electrically conductive reflector to the tensegrity structure so that the electrically conductive reflector attains the operating shape when the tensegrity structure is deployed to the deployed position. The method also includes deploying the tensegrity structure via an actuator.